Mechanized units are known that are moved through an existing trench in the ground for the purpose of laying a continuous length or lengths of a utility or utilities, such as conduits, pipes, cables, etc., or combinations thereof.
One type of mechanized unit exists for installing a utility surrounded by a select material at a predetermined position within an encasement of the select material in a trench. Where there is more than one utility line, the unit maintains a predetermined spatial relationship of the utilities with each other. Such a mechanized unit is propelled along the trench in which the flexible utility or utilities are being installed. The utility or utilities passes down through the unit by means of a guide device that supports the utility or utilities at a predetermined height above the trench bottom. As the utility or utilities pass through and/or have exited the guide device, the select material, located in the upper portion of the unit, gravitates in a generally vertical direction through the unit, onto and around the utility or utilities being installed.
There are drawbacks to this type of unit. The material as it gravitates on and around the utility or utilities being installed produces a vertical shear force on and can potentially damage the utility or utilities as they leave the control of the guide device which is above the bottom of the trench. Also, in this type of mechanized unit, the predetermined depth of select material that must end up, for protection, under the utility or utilities being installed must pass around and then must also flow underneath the utility or utilities to the bottom of the trench. This predetermined depth of select material can frequently not be guaranteed since most types of select material have varying flow and adhesive characteristics that change greatly with differing moisture contents. Obtaining the desired amount of select material required under the utility or utilities being installed is also made increasingly difficult as the diameter of the utilities become larger since this requires the select material to flow farther as it goes around and underneath the utility or utilities being installed.
In another type of mechanized unit, the utilities are to be placed at a desired depth in the trench on a bed of material, such as soil or concrete, of a desired height that is pre-laid on the bottom of the trench. As the utilities are placed on the bed, the unit maintains a desired spatial relationship between utilities in applications in which more than one utility to be laid in the trench. The utilities are laid on the bed by means of a guide device and the utility or utilities are then encased with a desired amount of material that is the same as or different from the bed material. The mechanized unit may be propelled through the trench by being connected directly to the device that digs the trench or by any other mechanical moving device, such as a tractor or other vehicle.
A unit of the latter type is disclosed in U.S. Pat. No. 6,478,508 granted on Nov. 12, 2002. The unit of this patent has a separate van, or hopper, for the each of the bedding and encasing materials. The unit is self-propelled or is towed along the trench as the bedding material is dispensed from one hopper to form the bed. The utility is laid on the bed and thereafter the encasing material is dispensed from the second hopper to encase the utility. This unit has the disadvantage that two hoppers are used and the two hoppers must be supplied with the materials. This means that there must be two separate sources of material supply, such as two vehicles that are moved along with the unit, or a single supply source that can feed two hoppers as the unit is moving. The latter configuration can present a problem in making certain that both hoppers are always adequately supplied with the material. Also, depending upon the depth of the bed and the depth of the encasement, the hoppers might have to be of different sizes and/or have the material supplied at different rates from the source or sources. This makes the material supply problem more complex and also gives rise to the possibility that one of the hoppers can become empty, which would interrupt the process of laying the utility or utilities. Also, the characteristics of the two materials in the two hoppers can be different so that there will not be a proper bonding or mixing of the material of the bed with the encasing material.